Visualization of anatomical data for disease diagnosis, surgical planning, or orientation during interventional therapy is an integral part of modern health care. However, only few medical imaging modalities are capable of providing real-time images of the patient's anatomy. A common procedure therefore involves the acquisition of static three-dimensional images, e.g. by means of computed tomography (CT) or magnetic resonance imaging (MRI) scanners, and subsequently manipulating and visualizing the acquired data on a computer, or radiological workstation.
For example, the data acquired from the CT or MRI scan of a patient's head may be used to generate a three-dimensional virtual model of the head and to display different views of the model. The computer may seemingly rotate the 3D virtual model of the head so that it can be seen from different angles, or may remove parts of the model so that hidden parts become visible, e.g. remove a part of the skull to inspect a tumor hidden underneath, or may highlight certain parts of the head such as soft tissue, so that those parts become more visible. Such techniques can assist a surgeon to decide upon the best point or direction from where to enter a patient's head to remove a tumor so as to minimize damage to the surrounding structure. They may also prove helpful for anatomy teaching.
However, in such conventional techniques it is usually the task of the physician to mentally transfer the three-dimensional virtual image to the patient, or in other words to establish a correspondence between the real object and the 3D virtual image generated from the static medical data. This not only requires considerable skill and experience, but is also prone to failures, which might have very serious consequences for the patient under therapy. In addition, navigation in the three-dimensional data set may not be as intuitive as desired and is often rather cumbersome. The inventors have repeatedly made the experience that surgeons sometimes require considerable time and training to get acquainted to new visualization software, and to find out which body part of the patient is currently displayed, or how to move to a different body part of the patient.
A system and method for mapping a three-dimensional virtual model of a body part to the real object is disclosed in United States Patent Application US2007/0018975 A1. The model is displayed on a computer screen and superimposed with an image of the real object taken by means of a video camera. The pose of the video camera in real space and/or the orientation of the 3D virtual object are varied until the virtual image is perfectly aligned with the real image. The pose of the camera in a real space may be tracked, and the orientation of the 3D virtual model may then be changed to follow the movement of the video camera. This can enable a physician to view subsurface structures from a perspective that corresponds to the current position or orientation of the video camera. The effect is a kind of “x-ray vision”, which allows the surgeon to see below the surface and into the patient. This may help the surgeon to operate on the patient with enhanced precision, and without having to mentally transfer the 3D virtual images shown on the computer screen to the patient placed before him.
However, the system disclosed in US2007/0018975 A1 requires means adapted to track the pose of the camera in real space, and hence can only be employed in specially equipped environments, e.g. in a specially equipped operating theatre. It further requires a careful initialization to guarantee that the coordinate system of the virtual 3D image is in perfect alignment with the coordinate system of the camera moving in real space. Only then does the view of the 3D virtual model correspond exactly to the orientation of the camera in real space. In addition, the patient is not allowed to move during inspection, for otherwise the coincidence of the coordinate system of the virtual model with the coordinate system of the camera in real space may be lost.